Filed of the Invention
The present invention relates to a fastening element and to a method for making a fastening element, and more specifically to a fastening element and method of making a fastening element having increased rigidity and increased strength in response to tensile forces. Furthermore, according to the method of the present invention, the fastening element is made in such a manner as to eliminate the formation of cracks and other weaknesses in the fastening element during fabrication.
Description of the Related Art
A fastening element has been known which is suitable for use as a locking device for a vehicle door, as well as for a link end of a conveyor chain, for example. The locking device comprises an engagement pawl disposed on the door side and a fastening element disposed on a vehicle body side. The engagement pawl is moved into a space defined by the fastening element under a locking operation, and is formed of a metallic material having a hook capable of being moved into or away from the space. On the other hand, the fastening element is formed of a metallic material having a U-shaped configuration and is fixed to the vehicle body through a center pole thereof.
Since the fastening element has a U-shaped configuration as described above, the engagement pawl is simply brought into contact or engagement with one point at the top of an engagement portion of the fastening element when a fastening operation of the fastening element is made. Thus, when the fastening element is continuously used over a long period of time, undesirable metal fatigue is concentratively induced at the contacted points of the fastening element as a result of the load imposed by the vehicle door. As a result, cracks are produced, thus causing a problem that the locked or fastened state between the engagement pawl and the fastening element cannot be ensured, thereby increasing the danger of the vehicle door becoming opened while the vehicle is running, especially when lateral loads are applied to the vehicle door while the vehicle is turning around a curve.
A metal heading machine for forming a metal head connecting two bolt sections, or legs, of a U-shaped shackle is known from U.S. Pat. No. 1,437,918. In this apparatus, a cylindrical metal rod having respective threaded ends is initially bent about a mandrel into a U-shaped blank. The legs of the U-shaped blank are then rigidly bound between the mandrel and respective lateral dies which are moved into engagement with the mandrel. While the legs of the U-shaped blank are rigidly bound in this fashion, a punch abruptly strikes the rounded portion of the U-shaped blank while the blank remains rigidly held in the dies. When the rounded portion of the U-shaped blank is abruptly struck by the punch, the rounded portion is forced outwardly to form ribs which extend beyond the position of the legs of the U-shaped blank, thereby forming the head of the shackle.
The apparatus according to U.S. Pat. No. 1,437,918, however, possesses the following disadvantages. Because the legs of the U-shaped blank are rigidly bound between the mandrel and the dies when the blank is struck by the punch, the legs cannot yield when the rounded portion of the blank is impacted and abruptly struck by the punch. Such rigid clamping of the legs between the dies causes counteracting forces to develop which act in opposition to the direction of impact when the punch strikes the rounded portion of the blank. As shown in FIG. 4, it has been discovered that such an action frequently produces small cracks C and other imperfections at the joints between the legs 14a, 14b and head portion 12 of the shackle, which detrimentally weaken the shackle. Thus, the shackle produced by this apparatus cannot meet the needs of fastening element having high strength and structural integrity.